1. Field of the Invention
The present invention relates to a device for setting the write laser power for writing a signal to a storage medium, such as an optical disk, by emitting a laser beam, and more particularly, to a write laser power setting device which sets the write laser power for writing frequency modulated luminance signal and chrominance signal in a video signal to an optical disk.
2. Description of the Prior Art
One type of device which writes a signal to a storage medium by emitting a laser beam is the laser writing device that writes a signal to an optical disk. The write laser power setting method applied in this conventional laser writing device is described below with reference to the accompanying drawings.
FIG. 8 is a block diagram of an optical disk recorder using the conventional write laser power setting method. The device comprises a laser drive circuit 4 to modulate the laser, a laser output adjusting circuit 5 that changes the laser power, a disk motor 7 to rotationally drive an optical disk 6, which is the data storage medium, an optical head 8 to write the modulated laser beam to the optical disk 6, a guide rail 9 to guide the movement of the optical head 8, an address read circuit 12 to detect the current read position of the optical head 8 from the signal output by the optical head 8, and a temperature detector 13 to detect the surface temperature of the optical disk 6.
FIG. 9 shows a graph of the relationship between laser power and the location of the optical head 8 on the optical disk 6, at different temperatures.
In a device (CAV) which drives the optical disk 6 at a constant velocity irrespective of the position of the optical head 8, the linear velocity of the optical head 8 relative to the optical disk 6 increases as the optical head 8 approaches the outside circumference of the optical disk 6, and the required write laser power therefore increases. Furthermore, because the optical disk is written by a laser, the required write laser power decreases as the surface temperature of the disk increases. As a result, in the laser writing device as shown in FIG. 8, the laser power adjusting circuit 5 is previously stored with data shown in FIG. 9. In other words, in the conventional laser writing device, the provisional laser power is corrected in two steps, which are: a temperature compensation effected by selecting one of different curves shown in FIG. 9; and location compensation effected by selecting different write laser power according to the selected curve.
However, the following problems are presented by the conventional write laser power setting device.
Due to various factors, such as the deviation of the laser generation characteristics of the laser generator mounted in the head 8, the dust or foreign matter covered on the surface of the laser emitting face, or wear away of the laser generator after a long term use, the optimum laser power as selected by the laser power adjusting circuit 5 may not be optimum laser power.
In addition, it is difficult to accurately detect the temperature of the write film of the optical disk, and there is a certain setting error in the laser power of this device when it is first set.
Therefore, variations in the write laser power, detection errors in the surface temperature, and variations in the laser power setting cause undesirable shifting from the optimum write laser power. Such an undesirable shifting causes bad influence on the written signal, particularly, when the written signal is a combined signal of FM luminance signal and FM chrominance signal wherein two or more different signals are frequency modulated with carriers of two or more frequencies, resulting in moire patterns on screen and thus deteriorate the image quality.